High-voltage equipment in the power grid runs a high risk of being subject to transient overvoltages caused by lightning strikes. In order to test the ability of electrical equipment to survive these harsh conditions, standards such as IEC60060 prescribe testing with a standardised simulated lightning overvoltage. This voltage is defined in IEC60060 as an ideal double-exponential waveshape with a front time of 1.2 µs and a time to half-value of 50 µs.
The team have developed their own software for measuring impulse voltages, which is used for both controlling a Yokogawa DL850 ScopeCorder and analysing the results. The analysis part is freely accessible at www.sp.se/lightning.
In actual testing, the impulse generator will often create a waveshape distorted with front oscillations at a higher frequency. Research has shown that these oscillations have a small impact on the electrical withstand of the test object, and IEC60060 specifies a standardised filter that conforms to the experimental results. The filter is defined as a digital filter to be applied on digitised measured data.
There are three major parameters to measure on a lightning impulse: peak voltage, front time and time to half-value. These need to be measured at very high voltages of 1000 kV and higher. The test object and the high-voltage divider are of considerable size - up to 10 or 20 m tall - and this, together with the requirement for a flat frequency response from DC into the megahertz range, presents significant challenges to both the design and calibration of the measurement equipment.
IEC60060 requires that the tests are performed with measuring equipment that has a traceable calibration. With equipment for these voltage levels, it is not generally feasible to transport it to a calibration laboratory, and so the Insititue has developed a test set-up, with the DL850 as an integral part, to bring the calibration laboratory to the customer.
By bringing a high-accuracy reference measuring system for voltage levels